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(2) corrosion resistance capabilities in a certain medium, as well as adequate toughness;

(3) having a good processing performance and welding inspection;

(4) In accordance with the different uses reasonable stability of the organization.

Heat resistant steel refers to work at high temperatures and corrosion resistance of the steel species having a certain strength and oxidation resistance, heat-resistant steel comprising a thermally stable steel and hot

Steel. Thermally stable steel is anti-oxidation at high temperature or the execution high temperature medium the corrosion without destroying steels, such as the furnace floor grate. They work when

The main failure mode of a high-temperature oxidation. Bear the load per unit area is not large. Heat resistant steel at high temperatures antioxidant capacity

And having adequate strength without causing large deformation or fracture types of steel, such as high temperature bolts, the turbine blade. They work required to bear a greater

The main reason for the load failure is high temperature strength is not enough to lead to instability, failure.

A steel thermal stability and thermal stability steel

Steel oxidation resistance and Ways to Improve

The workpiece and the high-temperature air, steam or gas contact surface to high temperature oxidation or corrosion damage. Therefore, the requirements of the workpiece must have compared

Good thermal stability. In addition to the added alloying element method, surface alloying methods, such as infiltration Cr infiltration Al or infiltration of Si to improve steel

The antioxidant properties.

Second, heat stabilized steel

The thermal stability of steel (also known as anti-oxidation steel) are widely used in industrial boilers components, furnace floor, muffle furnace, radiant tube, this uses heat

At room temperature, regardless of the mechanical properties of steel and the load times, but the strength of steel at high temperatures and the amount of deformation is not only time-related, and with

The temperature, which is heat-resistant steel, the so-called thermal strength. Resistance to plastic deformation and breaking heat intensity refers to heat-resistant steel in high temperature and load together

Bad capacity. This shows that in the assessment of the mechanical properties of materials under high temperature conditions must be assessed by the intensity of heat. Hot strength material at high temperature

The instantaneous performance under the conditions, and long-time performance.

The transient performance is often present Test of mechanical properties as measured by performance in high temperature conditions. Such as high-temperature tensile, high temperature shock and high

Temperature hardness. Which is characterized by high-temperature, short-term load Generally instantaneous performance steel hot strength of one side surface, the measured performance of a

Like not as design specifications, but as a reference index of select high-temperature materials.

Long performance refers to the measured performance of the material in common for a long time under the action of high temperatures and loads, a common performance indicators are: creep limit,

As the temperature increases, the heat-resistant steel continue to lower the resistance to plastic deformation and fracture, which is mainly caused by the following two factors:

(1) the influence factors of the softening of the heat-resistant steel. As the temperature increases, the steel between atoms binding force reduced atomic diffusion coefficient increases, thereby guide

Organizations to steel by metastable transition to a steady-state, multiphase alloy composition changes, such as the second phase of aggregation grew up, the sub-structure of alienation and occurred

Recrystallization and the like, these factors lead to a softening of the steel.

(2) deformation change of fracture mode. The deformation of a metallic material at low temperatures are generally to slipping method, but as the temperature increases, the carrier

The Dutch role longer, this time not only slip deformation and grain boundary sliding and migration, but also the proliferation of. The diffusion deformation is in the metal

Deformed but not see the slip line. This deformation mechanism at high temperatures increased thermal motion of atoms in the metal, resulting atom hair

HEALTH move, but under no external force movement of atoms is non-directional, so the macroscopic deformation does not occur; when an external force, the atom moves pole

Prone and directionality, thus promoting the deformation. Grain boundary can also occur when the temperature rises, under the external force, sliding and migration, the higher the temperature,

The load time is longer and more obvious, grain boundary sliding and migration.

Room temperature fracture of the metal in normal circumstances are generally transgranular fracture, which is due to the large degree of lattice distortion of the grain boundary region, the intragranular strength

Below due to the grain boundary strength. With the elevated temperature zone, the degree of lattice distortion due to the grain boundary area is small, so that atomic diffusion rate of increase in the grain boundary strength less

Weak. The higher the temperature, the load time is longer, then the metal fracture mode more rendered as intergranular fracture.

2. Improve the hot strength steel ways

Based on the above analysis, the main way to improve the hot strength of steel three areas: strengthening the matrix and the second phase strengthening, grain boundary strengthening.

(1) strengthening the matrix. The main starting point is to improve the matrix metal atom binding force, reducing the diffusion process of the solid solution. Studies have shown that

The chemical composition of the steel, any high melting point, since the diffusion coefficient is small, can increase the recrystallization temperature of the steel of the alloying elements dissolved in the matrix after they have

Improve hot strength steel. Such as Mo, W, Co and Cr. From the lattice of the solid solution type, austenitic group than ferrite substrate thermal strength

High. This is due to the The austenite matrix arrangement is compact, the diffusion process is not easy compared with the ferritic. If the iron-based alloy, Fe, C, Mo, etc. Element

Elements in A diffusion coefficient was significantly lower than in F diffusion coefficient, which makes the recovery and recrystallization process slows down, the slow-down in a second phase of aggregation speed,

So that the steel is not easy to soften in the hot state.

(2) In the second phase strengthening. The main point of departure is to request a second phase stability, not easy to gather grew up fine and uniform dispersion of long-term high temperature

State, so there are certain requirements on the composition and structure of the second phase particles. Most of the heat-resistant steel with refractory alloy carbides for strengthening phase, such as MC

M23C6, M6C. Higher heat resistance, and can be used with higher thermal stability intermetallic compound. Such as Ni3 (TiAl), Ni3Ti, Ni3Al

As strengthening of the matrix phase.

(3) grain boundary strengthening, to reduce the sliding of the grain boundary in the high temperature state, the following ways:

① reduce the grain boundaries, the need to appropriately control the grain size of the steel. Grains too fine grain boundary, although hindered intragranular slip, but the amount of deformation of the grain boundary sliding

Increases, reducing plastic deformation resistance. Grain is too large, the increased brittleness of the steel, it should be appropriate to control the grain size of the heat-resistant steel, usually 2 to 4 crystal

The granularity can get better high temperature performance.

(2) clean grain boundaries. The steel in the low-melting impurities such as S and P Yi in the grain boundary segregation, and easy to form low melting eutectic iron, thus weakening

Grain boundary strength steel heat intensity decreased. B and other rare earth elements can be added in the steel to form a stable compound of the high melting point, in the crystallization process

As nuclei, so that the fusible impurities transferred from the grain boundary to the grain interior, so that the grain boundary to be purified, and strengthen the grain boundary.

Other surface activation elements, can be filled with grain boundary vacancies, hinder the grain boundary diffusion of atoms to improve the creep resistance.

④ grain boundary precipitation strengthening. If not continuous reinforcing precipitated at the grain boundary phase, will allow the plastic deformation along the grain boundary sliding and crack along

Therefore, to further increase the use temperature is restricted. Such steel temperature, good heat resistance, thermal stability and process performance, the coefficient of linear expansion

Small number of lower aggregate amount of carbon, low prices, is suitable in 600 ~ 650 ℃ temperature zone of heat resistant steel, widely used in manufacturing boilers

Turbine and oil refining equipment.

Pearlitic heat resistant steel

Pearlitic heat resistant steel according to the aggregate amount of carbon and application characteristics of low-carbon pearlitic heat resistant steel can be divided into two categories in carbon pearlitic heat resistant steel, the former mainly

To the production of boiler tube, which is mainly used for the production of turbine and other heat-resistant fasteners, turbine rotor (including shaft, impeller), pearlitic heat resistant steel

Working temperature is not high, but because of long working hours, coupled by the corrosive effect of the surrounding medium, in the course of their work may produce the following

Organizational changes and performance changes.

1 pearlite spheroidization and carbide gathered. Pearlitic heat resistant steel, sheet carbide in the long-term high temperature into a ball,

Body and tempered the sorbite organizations, to ensure that in use temperatures organizations and performance stability. They are used in the manufacture of the like the temperature is below 588 ° C steamships

The machine and the blades of the gas turbine.

4 γ-Fe-based heat resistant steel

Pearlite, the general temperature of the martensitic heat resistant steel, can not apply to a higher use temperature is below 650 ℃ The reason is that, regardless of the

Pearlite matrix or heat resistant steel of martensitic matrix, the matrix phase are ferrite body, that is congenitally deficient. It is necessary to replace the base for the organization, with the Austrian

's Body. The reason why the austenite-based steel higher than the Fe-based steel having a heat resistance, the reason is that: the binding force between the γ-Fe lattice atoms than α-

The bonding force between the atoms of Fe lattice; γ-Fe diffusion coefficient is small; high γ-Fe the re-crystallization temperature (α-Fe recrystallization temperature of 450 to 600

℃, and γ-Fe recrystallization temperature greater than 800 ° C).

γ-Fe-based heat resistant steel has good weldability, oxidation resistance, high ductility and impact toughness. This kind of steel also has some disadvantages, such as the chamber

The low temperature yield strength, pressure processing and cutting poor, poor thermal conductivity, and from time to time changes in temperature effect force and therefore the thermal fatigue resistance is poor.

However, due to the high intensity of heat, so the full development and a wide range of applications.